1. Field of the Invention
The present invention relates generally to wind energy conversion systems in which kinetic energy of wind is converted into electric power and, more particularly, to wind energy conversion systems having blade assemblies carrying rotor elements for movement past stator elements to produce electric current.
2. Brief Discussion of the Related Art
Current wind power technology has primarily been developed by adapting or modifying non-wind technologies to wind power applications. This approach has resulted in wind power systems of excessive weight and cost, which has limited the cost-effectiveness and acceptance of wind power systems as a viable option for electric power production. As an example, a 500 KWe Vesta V39 wind power system typically weighs over 33 tons and costs more than $1,000,000 installed. The capital cost of such a system is around $2000 per KWe (about four times the capital cost of a coal plant), and the system weight translates to about 132 pounds per KWe. Consequently, the use of wind as a renewable energy source has not been taken full advantage of, and the wind power industry has not realized its full potential.
Current wind power technology typically utilizes “wind turbines”, which are in fact propellers normally of large diameter, i.e. 135 feet or more, and including two, three, four or five blades rotatable about a horizontal or nearly horizontal axis to effect rotation of a drive shaft. The propellers ordinarily rotate at extremely slow speeds due to their substantial mass and the centrifugal force at the blade roots. The drive shafts must be very large and very heavy, as represented by the following calculation of the size and weight needed for a solid steel drive shaft to transmit torque in a 500 KWe wind turbine system at 1 rpm.
                                          K            ⁢            We                    =                                    0.746              ×              torque              ×              rpm                                      5              ,              252                                      ;                                torque        =                                            5              ,              252              ×              K              ⁢                                                          ⁢              We                                      0.746              ×              rpm                                .                    Where rpm equals 1 and KWe equals 500,
  torque  =                    5        ,        252        ×        500                    0.746        ×        1              =                            2          ,          626          ,          000                0.746            =              3        ,        520        ,        107        ⁢                                  ⁢                  ft          .                      -                          ⁢                  lbs          .                    Assuming a yield strength of 10,000 psi for the solid steel drive shaft,
  d  =                    (                              (                          16              ×              torque                        )                    ÷                      (                          π              ×              10              ,              000              ⁢                                                          ⁢              psi                        )                          )                    1        3              =          12.148      ⁢                          ⁢              inches        .            Assuming a safety margin of 4 for fatigue, the diameter of the drive shaft needed is 19.284 inches, and this massive drive shaft must be rotated by the blades at low rpm. In addition, a drive shaft of this diameter is equivalent to 995 pounds per linear foot of the drive shaft.
Rotation of the drive shaft at low rotational speeds in prior wind turbine systems must be increased or stepped up in speed to about 900 to 3,600 rpm to drive a conventional generator. Increasing the drive shaft speed to drive a generator requires a large, costly and heavy gear step-up transmission assembly. The generator, weighing several tons, also contributes significant weight to the wind turbine system. An aerodynamic housing, such as the Nacelle, is commonly used in prior wind turbine systems to house equipment and typically weighs about 36,000 pounds. The excessive weight of conventional wind turbine systems necessitates a massive and costly tubular steel tower to support the propellers in an elevated position above the ground.
Conventional wind turbine systems commonly utilize positioning systems including computers and hydraulics to position the propellers to face into the oncoming wind and to “feather” the propellers, i.e. turn the propellers orthogonal to the wind in high wind conditions. One drawback to these positioning systems is that they shut down under the highest potential power output conditions.
Representative wind power systems are disclosed in U.S. Pat. No. 25,269 to Livingston, U.S. Pat. Nos. 1,233,232 and 1,352,960 to Heyroth, U.S. Pat. No. 1,944,239 to Honnef, U.S. Pat. No. 2,563,279 to Rushing, U.S. Pat. No. 3,883,750 to Uzzell, Jr., U.S. Pat. No. 4,182,594 to Harper et al, U.S. Pat. No. 4,398,096 to Faurholtz, U.S. Pat. No. 4,720,640 to Anderson et al, U.S. Pat. No. 5,299,913 to Heidelberg, U.S. Pat. No. 5,315,159 to Gribnau, U.S. Pat. No. 5,457,346 to Blumberg et al, U.S. Pat. No. 6,064,123 to Gislason, U.S. Pat. Nos. 6,278,197 B1 and 6,492,743 B1 to Appa, U.S. Pat. No. 6,504,260 B1 to Debleser, and U.S. Pat. No. 6,655,907 B2 to Brock et al, in U.S. Patent Application Publication No. US 2003/0137149 A1 to Northrup et al, and in German Patent DE 32 44 719 A1.
Only the Livingston patent discloses a blade assembly rotatable about a vertical axis of rotation. The blade assembly of the Livingston patent rotates a drive shaft and does not carry a rotor element for rotation past a stator element to produce electric current directly. Blade assemblies that carry rotor elements for rotation past stator elements to produce electric current are disclosed in the patents to Heyroth ('232 and '960), Honnef, Harper et al, Anderson et al, Gribnau, Gislason, and Brock et al, in the U.S. Patent Application Publication to Northrup et al and in the German patent, but the blade assemblies rotate about horizontal axes of rotation. The blade assembly of the Honnef patent comprises two counter-rotating wheels each having a rim carrying dynamo elements. The dynamo elements of one wheel rotate in opposition to the dynamo elements of the other wheel to produce electricity. The Honnef patent does not disclose two blade assemblies each capable of producing an electrical output independently. A wind power system having two counter-rotating blade assemblies in which each blade assembly carries rotor elements for rotation past stator elements is disclosed by Harper et al. Wind power systems having hoods for supplying air to the blade assemblies and having air intake openings facing lateral to the rotation axes of the blade assemblies are represented by the Livingston patent and the Brock et al patent.
In light of the foregoing, there is a need for a wind energy conversion system having two blade assemblies supported for rotation in opposite directions about a vertical rotation axis, with each blade assembly carrying a rotor for rotation past a stator to produce an electrical output directly and independently. There is also a need for a wind energy conversion system having two wind turbines with blade assemblies supported for rotation in opposite directions wherein the torques produced by the wind turbines are capable of being balanced to avoid a net torque on the tower. A further need exists for a wind energy conversion system having a blade assembly supported for rotation about a rotation axis, a hood disposed over the blade assembly having an air intake opening facing lateral to the rotation axis, and an exhaust plenum disposed beneath the blade assembly having an outlet opening, with the hood being rotatable about the rotation axis to maintain the air intake opening facing upwind and the exhaust plenum being rotatable about the rotation axis to maintain the outlet opening facing downwind. Another need exists for a wind energy conversion system having a blade assembly carrying a rotor for rotation past a stator to produce electric current, wherein the size of the air gap between the rotor and the stator is adjustable to control output current voltage in response to changes in rotational speed of the blade assembly.